Which is largely because horde suffer no consequences for launching an attack and getting wiped. If horde launch a full on assault on SH and 30 horde die, what consequences do they suffer? None.
If alliance launch an assault of 30 people at either IBGY or SFGY and simply get substantially delayed, not even wiped, they have lost SHGY and have no hard capped graveyards south of IWB choke and are now in a position from which it is extremely difficult to recover.
Exhibit A: Alliance Winning AV (On AV Weekends Only) - #130 by Graal-bloodsail-buccaneers